1. Field of the Invention
The invention relates to a method and apparatus for controlling the modulation current applied to a laser diode which is employed as a light transmitter in a transmission stage of a signal transmission system with a light wave guide.
2. The Prior Art
Digital light wave guide transmission systems as a rule incorporate gallium-aluminum arsenide laser diodes as optical transmitters, particularly at high transmission velocities. Such laser diodes have a bent light-current characteristic curve which is illustrated, for example, in FIG. 2b. The first or lowest bend or breakpoint of the light-current characteristic curve occurs given a current which is referenced as threshold current I.sub.t. A cut-in delay in the light transmission and a rectifier effect for the pulse signal current flowing through the laser diode are produced because of this threshold. Such a cut-in delay cannot be accepted, particularly in the transmission of signals with high pulse repetition frequencies. To eliminate this delay, the laser diode is supplied with a controlled preconduction or bias current Io which largely corresponds to the magnitude of the threshold current I.sub.t. It is known to employ the light emitted by the back mirror of the laser diode to control of the bias current Io supplied to the laser diode. Given different aging of the two mirrors, thus, repetitive errors of necessity ensue. An arrangement for controlling the pre-conductive current of a laser diode is described in the parallel German patent application Pat. No. 28 41 433.7. (U.S. Ser. No. 073,898)
In view of the constant optical pulse output often required, it is also necessary to vary and control the modulation current supplied to the laser diode based on temperature and age dependent on parameters of the laser diode.